DESCRIPTION

The
scanf()
family of functions scans input according to
format
as described below.
This format may contain
conversion specifications;
the results from such conversions, if any,
are stored in the locations pointed to by the
pointer
arguments that follow
format.
Each
pointer
argument must be of a type that is appropriate for the value returned
by the corresponding conversion specification.

If the number of conversion specifications in
format
exceeds the number of
pointer
arguments, the results are undefined.
If the number of
pointer
arguments exceeds the number of conversion specifications, then the excess
pointer
arguments are evaluated, but are otherwise ignored.

The
scanf()
function reads input from the standard input stream
stdin,
fscanf()
reads input from the stream pointer
stream,
and
sscanf()
reads its input from the character string pointed to by
str.

The
vfscanf()
function is analogous to
vfprintf(3)
and reads input from the stream pointer
stream
using a variable argument list of pointers (see
stdarg(3).
The
vscanf()
function scans a variable argument list from the standard input and the
vsscanf()
function scans it from a string; these are analogous to the
vprintf(3)
and
vsprintf(3)
functions respectively.

The
format
string consists of a sequence of
directives
which describe how to process the sequence of input characters.
If processing of a directive fails, no further input is read, and
scanf()
returns.
A "failure" can be either of the following:
input failure,
meaning that input characters were unavailable, or
matching failure,
meaning that the input was inappropriate (see below).

A directive is one of the following:

*

A sequence of white-space characters (space, tab, newline, etc.; see
isspace(3)).
This directive matches any amount of white space,
including none, in the input.

*

An ordinary character (i.e., one other than white space or aq%aq).
This character must exactly match the next character of input.

*

A conversion specification,
which commences with a aq%aq (percent) character.
A sequence of characters from the input is converted according to
this specification, and the result is placed in the corresponding
pointer
argument.
If the next item of input does not match the conversion specification,
the conversion fails---this is a
matching failure.

Each
conversion specification
in
format
begins with either the character aq%aq or the character sequence
"%n$"
(see below for the distinction) followed by:

*

An optional aq*aq assignment-suppression character:
scanf()
reads input as directed by the conversion specification,
but discards the input.
No corresponding
pointer
argument is required, and this specification is not
included in the count of successful assignments returned by
scanf().

*

An optional aqmaq character.
This is used with string conversions
(%s,
%c,
%[),
and relieves the caller of the
need to allocate a corresponding buffer to hold the input: instead,
scanf()
allocates a buffer of sufficient size,
and assigns the address of this buffer to the corresponding
pointer
argument, which should be a pointer to a
char *
variable (this variable does not need to be initialized before the call).
The caller should subsequently
free(3)
this buffer when it is no longer required.

*

An optional decimal integer which specifies the
maximum field width.
Reading of characters stops either when this maximum is reached or
when a nonmatching character is found, whichever happens first.
Most conversions discard initial white space characters (the exceptions
are noted below),
and these discarded characters don't count toward the maximum field width.
String input conversions store a terminating null byte (aq\0aq)
to mark the end of the input;
the maximum field width does not include this terminator.

*

An optional
type modifier character.
For example, the
l
type modifier is used with integer conversions such as
%d
to specify that the corresponding
pointer
argument refers to a
long int
rather than a pointer to an
int.

*

A
conversion specifier
that specifies the type of input conversion to be performed.

The conversion specifications in
format
are of two forms, either beginning with aq%aq or beginning with
"%n$".
The two forms should not be mixed in the same
format
string, except that a string containing
"%n$"
specifications can include
%%
and
%*.
If
format
contains aq%aq
specifications, then these correspond in order with successive
pointer
arguments.
In the
"%n$"
form (which is specified in POSIX.1-2001, but not C99),
n
is a decimal integer that specifies that the converted input should
be placed in the location referred to by the
n-th
pointer
argument following
format.

Conversions

The following
type modifier characters
can appear in a conversion specification:

h

Indicates that the conversion will be one of
d, i, o, u, x, X, or n
and the next pointer is a pointer to a
short int
or
unsigned short int
(rather than
int).

hh

As for
h,
but the next pointer is a pointer to a
signed char
or
unsigned char.

j

As for
h,
but the next pointer is a pointer to an
intmax_t
or a
uintmax_t.
This modifier was introduced in C99.

l

Indicates either that the conversion will be one of
d, i, o, u, x, X, or n
and the next pointer is a pointer to a
long int
or
unsigned long int
(rather than
int),
or that the conversion will be one of
e, f, or g
and the next pointer is a pointer to
double
(rather than
float).
Specifying two
l
characters is equivalent to
L.
If used with
%c
or
%s,
the corresponding parameter is considered
as a pointer to a wide character or wide-character string respectively.

L

Indicates that the conversion will be either
e, f, or g
and the next pointer is a pointer to
long double
or the conversion will be
d, i, o, u, or x
and the next pointer is a pointer to
long long.

q

equivalent to
L.
This specifier does not exist in ANSI C.

t

As for
h,
but the next pointer is a pointer to a
ptrdiff_t.
This modifier was introduced in C99.

z

As for
h,
but the next pointer is a pointer to a
size_t.
This modifier was introduced in C99.

The following
conversion specifiers
are available:

%

Matches a literal aq%aq.
That is,
%%
in the format string matches a
single input aq%aq character.
No conversion is done (but initial white space characters are discarded),
and assignment does not occur.

d

Matches an optionally signed decimal integer;
the next pointer must be a pointer to
int.

D

Equivalent to
ld;
this exists only for backward compatibility.
(Note: thus only in libc4.
In libc5 and glibc the
%D
is silently ignored, causing old programs to fail mysteriously.)

i

Matches an optionally signed integer; the next pointer must be a pointer to
int.
The integer is read in base 16 if it begins with
0x
or
0X,
in base 8 if it begins with
0,
and in base 10 otherwise.
Only characters that correspond to the base are used.

o

Matches an unsigned octal integer; the next pointer must be a pointer to
unsigned int.

u

Matches an unsigned decimal integer; the next pointer must be a
pointer to
unsigned int.

x

Matches an unsigned hexadecimal integer; the next pointer must
be a pointer to
unsigned int.

X

Equivalent to
x.

f

Matches an optionally signed floating-point number; the next pointer must
be a pointer to
float.

e

Equivalent to
f.

g

Equivalent to
f.

E

Equivalent to
f.

a

(C99) Equivalent to
f.

s

Matches a sequence of non-white-space characters;
the next pointer must be a pointer to character array that is
long enough to hold the input sequence and
the terminating null byte (aq\0aq), which is added automatically.
The input string stops at white space or at the maximum field
width, whichever occurs first.

c

Matches a sequence of characters whose length is specified by the
maximum field width
(default 1); the next pointer must be a pointer to
char,
and there must be enough room for all the characters
(no terminating null byte is added).
The usual skip of leading white space is suppressed.
To skip white space first, use an explicit space in the format.

[

Matches a nonempty sequence of characters from the specified set of
accepted characters; the next pointer must be a pointer to
char,
and there must be enough room for all the characters in the string, plus a
terminating null byte.
The usual skip of leading white space is suppressed.
The string is to be made up of characters in (or not in) a particular set;
the set is defined by the characters between the open bracket
[
character and a close bracket
]
character.
The set
excludes
those characters if the first character after the open bracket is a
circumflex
(^).
To include a close bracket in the set, make it the first character after
the open bracket or the circumflex; any other position will end the set.
The hyphen character
-
is also special; when placed between two other characters, it adds all
intervening characters to the set.
To include a hyphen, make it the last
character before the final close bracket.
For instance,
[^]0-9-]
means
the set "everything except close bracket, zero through nine, and hyphen".
The string ends with the appearance of a character not in the (or, with a
circumflex, in) set or when the field width runs out.

p

Matches a pointer value (as printed by
%p
in
printf(3);
the next pointer must be a pointer to a pointer to
void.

n

Nothing is expected; instead, the number of characters consumed thus far
from the input is stored through the next pointer, which must be a pointer
to
int.
This is
not
a conversion, although it can be suppressed with the
*
assignment-suppression character.
The C standard says: "Execution of a
%n
directive does not increment
the assignment count returned at the completion of execution"
but the Corrigendum seems to contradict this.
Probably it is wise
not to make any assumptions on the effect of
%n
conversions on the return value.

RETURN VALUE

These functions return the number of input items
successfully matched and assigned,
which can be fewer than provided for,
or even zero in the event of an early matching failure.

The value
EOF
is returned if the end of input is reached before either the first
successful conversion or a matching failure occurs.
EOF
is also returned if a read error occurs,
in which case the error indicator for the stream (see
ferror(3))
is set, and
errno
is set indicate the error.

ERRORS

EAGAIN

The file descriptor underlying
stream
is marked nonblocking, and the read operation would block.

EBADF

The file descriptor underlying
stream
is invalid, or not open for reading.

The result of an integer conversion would exceed the size
that can be stored in the corresponding integer type.

CONFORMING TO

The functions
fscanf(),
scanf(),
and
sscanf()
conform to C89 and C99 and POSIX.1-2001.
These standards do not specify the
ERANGE
error.

The
q
specifier is the 4.4BSD notation for
long long,
while
ll
or the usage of
L
in integer conversions is the GNU notation.

The Linux version of these functions is based on the
GNUlibio
library.
Take a look at the
info
documentation of
GNUlibc (glibc-1.08)
for a more concise description.

NOTES

The 'a' assignment-allocation modifier

Originally, the GNU C library supported dynamic allocation for string inputs
(as a nonstandard extension) via the
a
character.
(This feature is present at least as far back as glibc 2.0.)
Thus, one could write the following to have
scanf()
allocate a buffer for an input string,
with a pointer to that buffer being returned in
*buf:

char *buf;
scanf("%as", &buf);

The use of the letter
a
for this purpose was problematic, since
a
is also specified by the ISO C standard as a synonym for
f
(floating-point input).
POSIX.1-2008 instead specifies the
m
modifier for assignment allocation (as documented in DESCRIPTION, above).

Note that the
a
modifier is not available if the program is compiled with
gcc -std=c99
or
gcc -D_ISOC99_SOURCE
(unless
_GNU_SOURCE
is also specified), in which case the
a
is interpreted as a specifier for floating-point numbers (see above).

Support for the
m
modifier was added to glibc starting with version 2.7,
and new programs should use that modifier instead of
a.

As well as being standardized by POSIX, the
m
modifier has the following further advantages over
the use of
a:

*

It may also be applied to
%c
conversion specifiers (e.g.,
%3mc).

*

It avoids ambiguity with respect to the
%a
floating-point conversion specifier (and is unaffected by
gcc -std=c99
etc.).

BUGS

All functions are fully C89 conformant, but provide the
additional specifiers
q
and
a
as well as an additional behavior of the
L
and
l
specifiers.
The latter may be considered to be a bug, as it changes the
behavior of specifiers defined in C89.

Some combinations of the type modifiers and conversion
specifiers defined by ANSI C do not make sense
(e.g.,
%Ld).
While they may have a well-defined behavior on Linux, this need not
to be so on other architectures.
Therefore it usually is better to use
modifiers that are not defined by ANSI C at all, that is, use
q
instead of
L
in combination with
d, i, o, u, x, and X
conversions or
ll.

The usage of
q
is not the same as on 4.4BSD,
as it may be used in float conversions equivalently to
L.

EXAMPLE

To use the dynamic allocation conversion specifier, specify
m
as a length modifier (thus
%ms
or
%m[range]).
The caller must
free(3)
the returned string, as in the following example:

SEE ALSO

COLOPHON

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